Vector measurement of a magnetic field

ABSTRACT

A vectorial magnetometer ( 1 ), measures the components of a magnetic field in three directions (Oxyz) using a scalar magnetometer ( 2 ). The field is periodically modulated in each of the directions by generators (Gx, Gy, Gz) which have a specific frequency for each direction and that power coils (Ex, Ey, Ez). Synchronous demodulation of the of the output signal of the scalar magnetometer ( 2 ) for each of the three frequencies permits the relative continuous component of each axis to be found. The vectorial magnetometer ( 1 ) is characterized in that it has means (Dx D′x, Dy D′y, Dz D′z) that can carry out a double demodulation for phase and quadrature for each of the frequencies and processing means ( 70 ) that use the continuous component modules for phase and quadrature to calculate a transfer function of the scalar magnetometer at the frequency in question, and to apply this function to the correction of the components.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority based on International PatentApplication No. PCT/FR01/03212, entitled “Improved Vectorial Measurementof a Magnetic Field” by Jean-Michel Leger et al., which claims priorityof French Patent Application Serial No. 00/13328, filed on Oct. 18,2000.

TECHNICAL FIELD

This invention concerns a device and a procedure for measuring thecomponents of a magnetic field using a scalar magnetometer. It has anapplication in the measurement of small magnetic fields (of a grandeursimilar to that of the Earth's magnetic field, which is to say a fewtens of microteslas) that need to be obtained with great precision, ofaround 10⁻⁵.

STATE OF THE PRIOR TECHNIQUE

For a number of years, work has been consecrated to the vectorialmeasurement of magnetic fields using magnetometers that areintrinsically scalar, which is to say only sensitive to the magneticfield module, independently of its direction. The underlying idea behindthis work is to take advantage of the absolute nature of the scalarmeasurements (based on proton (RMN) or electron (RPE) resonance toovercome one of the main disadvantages of vectorial sensors, which is tosay their offsets and the associated low frequency derivatives.

Such a creation is described for example in patent No. FR-A-2 663 751 orin its American equivalent U.S. Pat. No. 5,221,897. These documentscontain, among others, bibliographical references on the subject.

A vectorial magnetometer using such a scalar magnetometer and capable ofsupplying the values of each of the components of the magnetic fieldvector, and whose module is provided by the scalar magnetometer isdescribed in the patent application FR 9807216.

According to the invention described in this patent application, thedevice for measuring the components of the magnetic field include amongothers a scalar magnetometer supplying an output signal whichcorresponds to the module of an applied magnetic field:

-   -   at least two conductor coils positioned around the said scalar        magnetometer, the axes of these conductor coils being orientated        in different directions (Ox, Oy, Oz).    -   means for powering each coil with a current with a determined        frequency (Fx, Fy, Fz) specific to this coil,    -   processing means that receive the signal provided by the scalar        magnetometer, these means being capable of carrying out        synchronous demodulations, at least at the frequencies (Fx, Fy,        Fz) of the currents powering the coils, these processing means        providing, for each frequency, a signal corresponding to the        component (Bx, By, Bz) of the magnetic field applied along the        axis (Ox, Oy, Oz) of the coil powered at this frequency.

The analysis of the signal induced by each of the coils permits thecomponent of the magnetic field vector of the module B_(o) projectedonto this axis to be found.

Furthermore, an article by J. M. G. MERAYO, F. PRIMDAHL AND P. BRAUERsubmitted on Jan. 21, 2000 to the participants of the first “Workshopfor calibration of the magnetic field of the Earth” held at Saclay (1)and entitled “the orthogonalisation of magnetic systems” explains how tochange from a real axial system, in principle orthonormal butnecessarily biased, which is to say orthonormal or close to anorthonormal system, to a truly reconstructed orthonormal axis system,and how to calculate the components of a magnetic field vector in theorthonormal axis system reconstructed from the measurements obtainedfrom the magnetic axes of the coils. A device capable of making such achange of axes may also include the means for calculating the axischange matrices, sensors dedicated to this function. The calculation mayalso be made from the readings made by each of the two other coils froma field emitted by one of the coils.

BRIEF DESCRIPTIONS OF THE INVENTION

Studies carried out by the inventor have revealed the presence of aresidue, which is to say a difference that is not nil, between thedirect measurement of the module value of the magnetic field vectorobtained by the scalar magnetometer and the calculated value of thismodule based on its three components, which are obtained by synchronousdemodulation at each of the field modulation frequencies.

FIG. 1 part A shows two curves. The first curve marked a represents theevolution in time of the module value of the magnetic field vectorobtained by the scalar sensor. A second curve shown in solid linesmarked b represents the evolution in time of this module valuecalculated from the three components obtained by the analysis of thesignals from each of the three coils.

FIG. 1 part B shows on an enlarged scale for the ordinates, theevolution in the same time of the difference dB between the modulevalues obtained by direct measurement and by calculation.

On this figure, the ordinates are graduated in nT and show a differencevarying between −15 nT and +10 nT. It is therefore concluded that themeasurement of the components using the method known in the prior artdoes not allow accuracy of the order of 10⁻⁵ to be obtained, and thatconsequently the measuring device and method need to be improved toreach this objective.

The invention therefore concerns a device and a procedure for measuringthe components of a magnetic field to around 10⁻⁵.

The hypothesis retained to improve the known process described in thepatent application FR 98 07216 mentioned above, is that the transferfunction f_(transfer) permitting the passage from the measurement to afrequency, for example Fx along the Ox axis to the field component inthis direction Ox, is equal to the product of a function of the saidfrequency Fx by a function depending on the measurement instrument used,which will be called f_(instrument).

The reason for this hypothesis is as follows. The operation in vectorialmode is based on the analysis of the response of the scalar magnetometerto a periodic mode stimulation at each of the frequencies Fx, Fy andpossibly Fz. The result is that any modification of the of thetransmission band of the scalar magnetometer affects the previouslydetermined transfer functions in a significant way when an accuracy ofaround 10⁻⁵ is sought.

The transmission band is a result of the different physical phenomenaused in the scalar probe. For a scalar magnetometer with optical heliumpumping, in compliance with the document FR-A-2 713 347 previouslymentioned (or its American equivalent U.S. Pat. No. 5,534,776) itdepends in particular on the relaxation time of the atoms in themetastable level which varies with the temperature proportionally to1/√T and with the current of the electrical field with high frequencydischarge maintenance, of the pumping time which depends on theintensity of the laser beam, or even the time constant characteristic ofthe magnetic resonance which is a function of the radio-frequency field.

Therefore, according to the invention, it is accepted that:f _(instrument) *H(F)=f _(transfer)

The instrumental function f_(instrumental) which depends on the knowncharacteristics of the instrument is known to within 10⁻⁵. According tothe invention, the transfer function H(F) remains to be determined foreach frequency Fx, Fy or Fz.

If an initial estimation is made that the transmission band of thescalar sensor is similar to a low pass filter of the first order:${H(F)} = \frac{1}{1 + i_{F_{o}}^{F}}$

where F_(o) is the filter cut off frequency.

The module of this transfer function can therefore be described by:${{H(F)}} = \frac{1}{\sqrt{1 + ( \frac{F}{F_{o}} )^{2}}}$

In the case of a small frequency preceding the cut off frequency of thisfilter, the module is developed in the second order:${{H(F)}}\underset{F{\operatorname{<<}F_{o}}}{\cong}{1 - {\frac{1}{2}( \frac{F}{F_{o}} )^{2}} + {o( \frac{F_{o}}{F} )}^{4}}$

giving a relative fluctuation proportional to the relative fluctuationsof F_(o): $\begin{matrix}{\frac{\Delta{H}}{H}\underset{F{\operatorname{<<}F_{o}}}{\cong}{\frac{( \frac{F}{F_{o}} )^{2}}{1 - {\frac{1}{2}( \frac{F}{F_{o}} )^{2}}}\frac{\Delta\quad F_{o}}{F_{o}}}\underset{F{\operatorname{<<}F_{o}}}{\cong}{( \frac{F}{F_{o}} )^{2}\frac{\Delta\quad F}{F_{o}}}} & (4)\end{matrix}$

The fluctuations of these transfer functions are therefore proportionalto those of the transmission band of the helium scalar sensor, with theproportionality factor depending on F². Consequently, the scalar residuevaries proportionally to the fluctuation of the transmission band:${\Delta( {dB}_{à} )} \cong {\Delta( {\sqrt{\sum\limits_{{i = x},y,z}\quad( {{H( F_{i} )}\frac{B_{o}H_{li}}{b_{mi}}} )}}^{2} )} \cong {2\frac{\sum\limits_{{i = x},y,z}{\frac{B_{o}H_{li}}{b_{mi}}{H( F_{l} )}( \frac{F_{li}}{F_{o}} )^{2}1}}{\sum\limits_{{i = x},y,z}\quad( {{H(F)}\frac{B_{o}H_{li}}{b_{mi}}} )^{2}}\frac{\Delta\quad F_{o}}{F_{o}}}$

Furthermore, the phase of this filter is given by the expression:${\varphi(F)} = {- {\arctan( \frac{F}{Fo} )}}$

For a frequency close to the continuum, we can write:${\varphi(F)} = {{- \frac{F}{Fo}} + {o( \frac{F}{F_{o}} )}^{3}}$

For F<<F_(o), it can be deduced that a fluctuation ΔFo of the band ofthe scalar sensor therefore gives rise to a fluctuation Δφ of the phasesuch that:$\frac{\Delta\varphi}{\varphi}\underset{F{\operatorname{<<}F_{o}}}{\cong}{- \frac{\Delta\quad F_{o}}{F_{o}}}$

By examining the parallel evolutions of the scalar residue dB and thephases of the harmonics (as the third projection in the experimentalconfiguration retained was almost nil, the corresponding signal phasehas not been shown) during the recording shown in FIG. 2, it can beremarked that the phases φ_(x) and φ_(y) of the two components x and yfluctuate in correlation with the evolutions of dB, shown in FIG. 1 incompliance with the modelling results.

The proposed method consists of taking into account the fluctuations ofthe transmission band of the scalar magnetometer by using theinformation contained in the signal phases generated by one or more ofthe modulations.

However, instead of a single synchronous detection for each modulationas in the patent application FR 98 07216 previously mentioned, a doublesynchronous detection in phase and in quadrature is carried out, whichmakes possible the simultaneous detection of the amplitude M and thephase φ of each signal in function of the modules in phase P and inquadrature Q, M=(P²+Q²)^(1/2) and φ=Arct (Q/P).

The phase measurement makes possible an estimation of the cut offrequency F_(o) of the magnetometer which is then used to correct thetransfer function module H(F) at the frequency in question.

Thus a device for measuring the magnetic field components, improvedaccording to the invention, will include all of the elements describedin relation with the patent application FR 98 07 216 previouslymentioned, but with at least one of the means to apply to at least oneof the coils Ex, Ey, Ez, a first signal that will be modified togenerate a second signal at the same frequency but offset in relation toone another. As in the prior art, the first signal will be applied tothe coils Ex, Ey, Ez.

The detection means associated to each of the coils will be capable ofnot only detecting in phase, as in the prior art, but also in compliancewith the invention, detecting in quadrature. To this end, they willreceive the second signals offset with respect to the first signals. Thecalculation means will include, in compliance with the invention, meansfor calculating the phase of the scalar magnetometer transfer functionfor at least one of the frequencies applied to a coil, and the module ofthis function.

In summary, the invention concerns a precision device for measuring thecomponents of the magnetic field using a scalar magnetometer supplyingan output signal corresponding to the module of an applied magneticfield and comprising:

-   -   at least two conductor coils positioned around the said scalar        magnetometer, the axes of these conductor coils being orientated        in different directions (Ox, Oy, Oz).    -   means for powering each coil with a current with a determined        frequency (Fx, Fy, Fz) specific to this coil,    -   processing means that receive the signal provided by the scalar        magnetometer, and processing this signal to deduce the value of        the continuous component along each of the axes of the coils.    -   device characterised in that at least one of the means that        power each coil with a current of a determined frequency (Fx,        Fy, Fz) specific to each coil, produce at this frequency signals        in phase and in quadrature, in that the processing means which        receive the signal provided by the scalar magnetometer are        capable of carrying out, apart from a synchronous demodulation        in phase at least at the frequencies (Fx, Fy, Fz) powering the        coils, a demodulation in quadrature for at least one of the        frequencies (Fx, Fy, Fz) of the currents powering the coils (Ex,        Ey, Ez), these demodulation means receiving, apart from the        signal in phase generated by the power supply of the coils (Ex,        Ey, Ez), the signal in quadrature from the generation means (Gx,        Gy, Gz) and finally in that these processing means include a        calculation module that provides, based on the results of the        different demodulations, for each frequency, a signal        corresponding to the corrected component (Bxc, Byc, Bzc) of the        magnetic field applied along the axis (Ox, Oy, Oz) of the coil        (Ex, Ey, Ez) powered at this frequency.

In practice, a device of the prior art could be modified by adding to atleast one of the generators (Gx, Gy, Gz) to supply each coil with acurrent of a determined frequency (Fx, Fy, Fz) specific to this coil, aquarter of a wave delay. Therefore one coil, Ex for example, would bepowered by a direct output from the generator. However, the demodulationmeans will receive, apart from the direct output, a parallel output inquadrature.

The software programmes of the processing means will be modified on theone hand to carry out a synchronous demodulation, in phase and inquadrature, on at least one of the specific frequencies of each axis andto calculate a correction of the value of the continuous component alongeach of the axes, taking into account the results of this doubledemodulation.

In one embodiment, there is a coil for each axis, and three generatorsGx, Gy and Gz. These three generators are each equipped with a quarterwave delay. Each of the three generators is coupled to a coil and ademodulator carrying out a phase demodulation. Each of the delays iscoupled to a demodulator carrying out a quadrature demodulation.

It is also possible to use only two quarter wave delays, and tocalculate the transfer function solely for the frequencies respectivelypowering the coil Ex and the coil Ey, the transfer function for thethird component being obtained indirectly from one of the two transferfunctions obtained directly, preferably the one with the largest module.

The invention also concerns a process for measuring the components of amagnetic field along the axes (Ox, Oy, Oz), orientated in differentdirections, obtained using a device using a scalar magnetometerproviding an output signal corresponding to the module B_(o) of anapplied magnetic field, the process consisting of superposing on themagnetic field to be measured, fields that are each orientated alongeach of the axes (Ox, Oy, Oz) respectively, these superposed fieldsvarying in time periodically according to a frequency (Fx, Fy, Fz) thatis specific to each axis, and then synchronously demodulating at each ofthe frequencies (Fx, Fy, Fz) the signal emitted from the scalarmagnetometer and processing the signal resulting from a synchronousdemodulation at one frequency, in order to deduce the value of thecontinuous component of the magnetic field along the axis receiving thesuperposed field at this frequency, a process characterised in that forat least one of the axial directions, the demodulation is carried outsynchronously in phase and in quadrature for at least one of thesuperposed fields, and in that from the demodulation results at leastone transfer function of the magnetometer is directly calculated for oneof the frequencies at which a periodic field was superposed at thisfrequency, and possibly indirectly one or two transfer functions from atransfer function obtained directly and in that the value of the moduleof each component is corrected by application of one of the transferfunctions calculated directly or indirectly.

Thus in general, a transfer function will be calculated directly orindirectly at each of these frequencies and each component will becorrected according to the transfer function calculated at the frequencycorresponding to this component.

According to one embodiment of the process:

-   -   the uncorrected components of the magnetic field are calculated;    -   a comparison is made of the components to determine which is the        largest;    -   the correction is made by applying to each axial component the        transfer function corresponding to the frequency modulation        applied to the coil along the axis of this axial component, the        transfer functions of the largest component being obtained        directly from the demodulation results, and the transfer        function of the other components being obtained indirectly from        the transfer function of the largest component.

According to one advantageous variant, a check will be made that thesignals leading to the calculation of each transfer function accordingto the frequency have acceptable signal noise level ratios. In practice,the signal noise level will be acceptable if the level of the signal ishigher than a predetermined threshold. Consequently, if the level isgreater than this threshold, then the transfer function that iscalculated directly for this frequency will be used. However, if thelevel of one of the signals is lower than this threshold, then thetransfer function obtained directly at this frequency will be replacedby a transfer function obtained indirectly, which is to say calculatedfrom a transfer function obtained directly for another frequency,preferably the frequency corresponding to the continuous component withthe highest value.

We have seen earlier in an example relating to a low pass filter of thefist order, that the fluctuations of the transfer functions areproportional to those of the transmission band of the helium scalarsensor, the proportionality factor depending on F². It is thereforepossible to calculate a transfer function at a second frequency if atransfer function is known for a first modulation frequency.

An example of such a calculation is given by: the formula (4) at thebottom of page 6 earlier in this document.

It should be noted that the cut off frequencies of the transferfunctions are situated between 200 and 1,000 Hertz and in general around400 Hertz, whereas the superposition field frequencies are chosenbetween 5 and 60 Hertz so that the values of these frequencies arealways smaller than the cut off frequencies. The result is that thecalculations may be simplified as indicated above, on pages 5 to 7, inparticular by making developments limited for example to the firstorder. Also, as noted just above, the transfer functions at thedifferent modulation frequencies may be calculated from one of them.This is why, when the demodulation data at a frequency has interference,it is better for that for this frequency with interference, that atransfer function be calculated based on a transfer function obtainedfor a different frequency in which the signal noise ratio is high.

Finally, sinusoidal periodic figures are preferably applied to thecoils.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 which as already been described comprises parts A and B. FIG. 1part A shows two curves. The first curve marked a represents theevolution in time of the module value of the magnetic field vector. Asecond curve marked b represents the evolution in the same time of thismodule value calculated from its three components. Part B represents theevolution in time of the value of the difference dB between the valuesof the two modules represented in part A.

FIG. 2 which has already been described shows during the same time asthe curves represented in FIG. 1 the curves of the evolution in time ofthe phase value of the two transfer functions corresponding to twosuperposition frequencies.

FIG. 3 illustrates an embodiment of a device according to the invention.

DESCRIPTION OF A SPECIFIC EMBODIMENT

It can be seen, first of all in FIG. 3, a scalar magnetometer 2 withoptical helium pumping, in compliance with the document FR-A-2 713 347previously mentioned (or its American equivalent U.S. Pat. No.5,534,776).

This magnetometer 2 comprises in a known manner a cell 10 filled withhelium, a laser 14 emitting a beam 15, a polariser 16 supplying arectilinear polarised beam 17, a photo-sensor 24 that receives: the beam18 after it has passed through the cell 10, a frequency servo circuit21, a radio-frequency generator 22, a frequency meter 26 and a dischargecircuit 30. The generator 22 powers a coil 56 positioned close to thecell 10 in order to create a radio-frequency field in the latter. Thecoil 56 and the polariser 16 are mechanically held together, so that anyrotation applied to the polariser causes a rotation of the same angle inthe direction of the field, the intensity of the latter being defined bythe generator 22.

Advantageously, to connect the means 26 and 16, a rotating contact isused, for example a capacity coupling contact or by a transformer whoseprimary coil is fixed and the secondary coil is mobile.

In preference, the coil 10 and the polariser 16 are mounted so that thepolarisation and the field applied are parallel.

The circuit 40 is a servo circuit which controls a motor 46 that adjuststhe angular position of the polariser 16. This circuit is described inthe document mentioned.

In the embodiment illustrated, the device comprises three conductorcoils, Ex, Ey and Ez, whose axes form a trihedron that is in principletrirectangular Oxyz. To make this easier to understand, these coils areshown distant from the cell 10 but, it should be understood that thesecoils surround the cell 10. Each of the coils Ex, Ey, Ez is connected toa generator, respectively Gx, Gy and Gz, each set to a specificmodulation frequency F_(x), F_(y), F_(z). The modulation frequenciesF_(x), F_(y), F_(z) are of course different from one another. Incompliance with the invention, each generator Gx, Gy and Gz, is equippedwith a quarter wave delay 31, 32, 33 respectively.

A processing circuit 70 comprises six demodulators, Dx, D′x, Dy, D′y,Dz, D′z, coupled respectively to the generators Gx, Gy and Gz and to thequarter wave delays 31, 32, 33 and which receive the output signal sentby the scalar magnetometer, which is to say, in the embodimentillustrated, the signal provided by the frequency meter 26. This signalis generally a digital signal providing the module Bo of the field to bemeasured.

The application of a external field Hx applied along the Ox axis at afrequency Fx modifies the module of the field to be measured by aquantity that varies at the frequency Fx. The output signal of thescalar magnetometer, which provides the field module, therefore containsa component that varies at the frequency Fx, the component representingthe projection of the field to be measured on the Ox axis. The value ofthis projection depends among others as explained earlier in thisdocument on the value of the transfer function of the magnetometer forthis frequency. By demodulating the scalar magnetometer output signal inphase and in quadrature at the frequency Fx, then the value of thiscomponent can be identified and also the value of the module and thephase of the transfer function. It is thus possible to calculate acorrected value of the component on Ox. The same also applies for thetwo other components.

The demodulation part of the circuit 70 composed of the circuits Dx,D′x, Dy, D′y, Dz and D′z is typical and can operate either bymultiplication and demodulation or by Fournier's fast transformation(FFT). It supplies the phase and quadrature Bx, B′x, By, B′y, Bz, B′z ofthe field to the components in the three directions Ox, Oy and Oz. Thesecomponents are received in a calculation module 34 of the processingcircuit 70. The module 34 calculates the transfer function for eachfrequency and corrects the value of the component for each axisdepending on this transfer function to provide corrected components.

Advantageously, the calculation module 34 incorporates an axis changefunction to provide the values of each component of the field to anorthonormal trihedron calculated using one of the methods described byJ. M. G. MERAYO, F. PRIMDAHL AND P. BRAUER already mentioned. Ingeneral, as explained in this article, an axis of the calculatedorthonormal trihedron is merged with one of the axes of the coils. Inthe example described here, the parameters of the transformationmatrices are calculated in a prior calibration phase identical inprinciple to the methods applied to the flux gates).

When the three corrected field components Bx, By and Bz are thoserecalculated in the calculated orthonormal identification, they arelinked to the field module Bo by the relationship:B _(x) ² +B _(y) ² +B _(z) ² =B _(o) ²

It is also possible to use just two coils, for example Ex and Ey, twogenerators Gx and Gy and two demodulators Dx and Dy, to obtain the twocomponents Bx and By and to find the third component Bz from thecontinuous value Bo given by the frequency meter 26. The relationshipthat gives Bz is obviously:|Bz|=[B _(o) ² −B _(x) ² −B _(y) ²]^(1/2)

Naturally, the embodiment that we have just described is only given byway of example and another scalar magnetometer than the one describedcould be used, the essential aspect being that it provides an outputsignal reflecting the field module.

It is also possible to use just one quarter wave delay, for example 31,and to calculate the transfer function solely for the frequency poweringthe coil Ex, the transfer functions for the other two components beingobtained indirectly from the sole transfer function obtained directly.

It has been shown above that the number of coils Ex, Ey and Ez can varybetween two and three, that the number of quarter wave delays 31, 32,32, can vary between one and three, and the result is that the number ofsynchronous detectors can vary between three and six. There are three ofthem if there are two generators and only one with a delay, and six forthree generators each equipped with a delay.

It is better for the processing means 70 to have a number of synchronousdetection means equal or at most equal to the total number of signalgenerators and the number of quarter wave delays 31-33 equipping: thesaid signal generators.

(1) The article by J. M. G. MERAYO, F. PRIMDAHL, AND P. BRAUER submittedon Jan. 21, 2000 to the participants of the first “Workshop forcalibration of the magnetic field of the Earth” held at Saclay andentitled “the orthogonalisation of magnetic systems” should be publishedin the magazine “Sensors and actuators”.

1. Precision device (1) for measuring the components of the magneticfield comprising: a scalar magnetometer (2) supplying an output signalcorresponding to the module of an applied magnetic field: at least twoconductor coils (Ex, Ey, Ez) positioned around the said scalarmagnetometer, the axes of these conductor coils (Ex, Ey, Ez) beingorientated in different directions (Ox, Oy, Oz), means for powering eachcoil with a current with a determined frequency (Fx, Fy, Fz) specific tothis coil, processing means (70) that receive the signal provided by thescalar magnetometer (2), and processing this signal to deduce the valueof the continuous component along each of the axes of the coils, devicecharacterised in that at least one of the means (Gx, Gy, Gz) that powereach coil (Ex, Ey, Ez) with a current of a determined frequency (Fx, Fy,Fz) specific to each coil, produce at this frequency signals in phaseand in quadrature, in that the processing means (70) which receive thesignal provided by the scalar magnetometer (2) are capable of carryingout, apart from a synchronous demodulation in phase at least at thefrequencies (Fx, Fy, Fz) powering the coils, a demodulation inquadrature for at least one of the frequencies (Fx, Fy, Fz) of thecurrents powering the coils (Ex, Ey, Ez), these demodulation meansreceiving, apart from the signal in phase generated by the power supplyof the coils (Ex, Ey, Ez), the signal in quadrature from the generationmeans (Gx, Gy, Gz) and finally in that these processing means (70)include a calculation module (34) that provides, based on the results ofthe different demodulations, for each frequency, a signal correspondingto the corrected component (Bxc, Byc, Bzc) of the magnetic field appliedalong the axis (Ox, Oy, Oz) of the coil (Ex, Ey, Ez) powered at thisfrequency.
 2. Precision device (1) for measuring the components of themagnetic field of claim 1, in which the axes (Ox, Oy, Oz) of theconductor coils (Ex, Ey, Ez) form an orthonormal reference or close toan orthonormal reference.
 3. Device (1) of claim 2, comprising threeconductor coils (Ex, Ey, Ez) at mutually orthogonal axes (Ox, Oy, Oz).4. Device (1) of claim 1, in which the generation means (Gx, Gy, Gz)that power each coil (Ex, Ey, Ez) with a current of a determinedfrequency (Fx, Fy, Fz) specific to this coil are equipped with a quarterwave delay (31-33) whose output is coupled to a detection means (D′x,D′y, D′z).
 5. Device (1) of claim 2, in which the generation means (Gx,Gy, Gz) that power each coil (Ex, Ey, Ez) with a current of a determinedfrequency (Fx, Fy, Fz) specific to this coil are equipped with a quarterwave delay (31-33) whose output is coupled to a detection means (D′x,D′y, D′z).
 6. Device (1) of claim 3, in which the generation means (Gx,Gy, Gz) that power each coil (Ex, Ey, Ez) with a current of a determinedfrequency (Fx, Fy, Fz) specific to this coil are equipped with a quarterwave delay (31-33) whose output is coupled to a detection means (D′x,D′y, D′z).
 7. Device (1) of claim 3, in which the processing means (70)have a number of synchronous detection means equal to the total numberof signal generators and the number of quarter wave delays (31-33)equipping the said signal generators.
 8. Process for measuring thecomponents of a magnetic field along the axes (Ox, Oy, Oz), orientatedin different directions, obtained using a device using a scalarmagnetometer providing an output signal corresponding to the module B,of an applied magnetic field, the process consisting of superposing onthe magnetic field to be measured, fields that are each orientated alongeach of the axes (Ox, Oy, Oz) respectively, these superposed fieldsvarying in time periodically according to a frequency (Fx, Fy, Fz) thatis specific to each axis, and then synchronously demodulating at each ofthe frequencies (Fx, Fy, Fz) the signal emitted from the scalarmagnetometer and processing the signal resulting from a synchronousdemodulation at one frequency, in order to deduce the value of thecontinuous component of the magnetic field along the axis receiving thesuperposed field at this frequency, a process characterised in that forat least one of the axial directions, the demodulation is carried outsynchronously in phase and in quadrature for at least one of thesuperposed fields, and in that from the demodulation results at leastone transfer function of the magnetometer is directly calculated for oneof the frequencies at which a periodic field was superposed at thisfrequency, and possibly indirectly one or two transfer functions from atransfer function obtained directly and in that the value of the moduleof each component is corrected by application of one of the transferfunctions calculated directly or indirectly.
 9. Process for measuringthe components of a magnetic field of claim 8 in which, based on thedemodulation results in phase and in quadrature, a transfer function permodulation frequency is calculated directly and in which the calculatedtransfer function is applied for each of the modulation frequenciesrespectively for the correction of the component of the magnetic fieldcorresponding to this modulation frequency.
 10. Process for measuringthe components of a magnetic field of claim 8 in which; the uncorrectedcomponents of the magnetic field are calculated; a comparison is made ofthe components to determine which is the largest; the correction is madeby applying to each axial component the transfer function correspondingto the frequency modulation applied to the coil along the axis of thisaxial component, the transfer function of the largest component beingobtained directly from the demodulation results, and the transferfunction of the other components being obtained indirectly from thetransfer function of the largest component.
 11. Process for measuringthe components of a magnetic field of claim 8 in which: the uncorrectedcomponents of the magnetic field are calculated; the value of each ofthe components is compared to a threshold value; a comparison is made ofthe components to determine which is the largest; the correction is thenmade to each of the components above the threshold by applying thetransfer function corresponding to the frequency relative to thiscomponent, obtained directly, and the components below the threshold arecorrected by applying a transfer function obtained indirectly from thetransfer function of the largest component.
 12. Process for measuringthe components of a magnetic field of claim 8 in which the components ofthe magnetic field are recalculated based on axial components correctedin a fictitious exactly orthonormal axis system, one of the fictitiousaxes coinciding with one of the coil axes.
 13. Process for measuring thecomponents of a magnetic field of claim 9 in which the components of themagnetic field are recalculated based on axial components corrected in afictitious exactly orthonormal axis system, one of the fictitious axescoinciding with one of the coil axes.
 14. Process for measuring thecomponents of a magnetic field of claim 10 in which the components ofthe magnetic field are recalculated based on axial components correctedin a fictitious exactly orthonormal axis system, one of the fictitiousaxes coinciding with one of the coil axes.
 15. Process for measuring thecomponents of a magnetic field of claim 11 in which the components ofthe magnetic field are recalculated based on axial components correctedin a fictitious exactly orthonormal axis system, one of the fictitiousaxes coinciding with one of the coil axes.